Surface nanostructuring by grafting functional polymers to a substrate surface has been used to enhance chemical functionality and to alter the surface topology of native inorganic and organic materials. For example, graft-polymerized ethylenically unsaturated monomers offers unique properties in applications such as micropatterning in electronics fabrication, adhesion in carbon fibers and rubber dispersions, and as selective layers in fuel cells and separation membranes. Organic and inorganic surfaces modified with grafted polymers have demonstrated anti-fouling characteristics in separation membranes, high chemical selectivity in chemical sensors, and surface lubricating properties. In such applications, the grafted polymer phase, composed of nanoscale, single-molecule chains covalently and terminally bound to a substrate or surrogate surface, serves to impart unique material properties to the substrate while maintaining the chemical and physical integrity of the native surface. Moreover, the grafted chains remain attached to the surface even when exposed to a solvent in which the polymer is completely miscible.
A tethered polymer phase can be formed either by polymer grafting (“grafting to”) or graft polymerization (“grafting from”). Surface chain coverage and spatial uniformity achieved by polymer grafting may be limited by steric hindrance. In contrast, graft polymerization, which is the focus of the present invention, proceeds by sequential monomer addition, thereby allowing for the formation of a denser surface coverage.
Structuring surfaces with grafted vinyl monomers and other ethylenically unsaturated monomers is commonly achieved by free radical graft polymerization (FRGP), where the polymer chain size, chain length uniformity, and surface density are dictated by the initial monomer concentration, reaction temperature and density of the surface immobilized initiators or initiators in solution. However, broad molecular weight chain size distributions resulting from uncontrolled macroradical reactions in solution and limitations in surface density due to the restriction of pre-grafted surface initiation sites make this approach unattractive for nanoscale-engineered polymer surface architectures.
Free radical polymerization relies on initiator species to initiate either solution polymerization, in which polymers grown in solution may bind to reactive surface sites by polymer grafting, or surface polymerization, in which monomers undergo direct surface grafting from immobilized surface initiators (e.g., surface-grafted reactive groups) or surface monomers (e.g., ethylenically unsaturated monomers) by graft polymerization (e.g., surface grafted reactive groups). However, the occurrence of competitive polymer chain grafting, chain transfer reactions, and surface chain growth by propagation result in a polydisperse grafted polymer chain size in contrast to the more uniform surface chain size that is achieved by grafting of preformed polymer chains of a uniform size. Further, for inorganic substrates, the density of grafting sites for graft polymerization is limited by the availability of surface hydroxyl groups on the oxide surface, which serve as anchoring sites for surrogate surface initiators and macroinitiators. For example, the surface concentration of hydroxyl groups on fully hydrolyzed silica and zirconia are 7.6 μmoles/m2) (4.6 molecules/nm2) and 5.6-5.9 μmoles/m2 (3.4-3.6 molecules/nm2), respectively.
In recent years, the demand for sophisticated, advanced materials for nanoscale devices has led to a growing interest in controlled radical polymerization (CRP), whereby grafted polymer domains may be precisely structured by controlling polymer chain growth and grafted chain polydispersity. CRP utilizes a control agent that reversibly binds to the surface-bound macroradical chain, establishing a thermodynamic equilibrium that favors the capped polymer in the dormant phase. The presence of the control agent limits the number of “live” chains in solution, thus enabling control over the rate of surface polymerization while reducing chain termination. Controlled polystyrene graft polymerization, with number-average molecular weights (Mn) and polydispersity indices (PDI), has been reported for the following CRP methods: atom transfer radical graft polymerization (ATRGP) (Mn=10,400-18,000 g/mol and PDI=1.05-1.23), reversible addition-fragmentation chain transfer (RAFT) graft polymerization (Mn=12,800-20,000 g/mol, PDI=1.10-1.40), and nitroxide-mediated graft polymerization (NMGP) (Mn=20,000-32,000 g/mol, PDI=1.20-1.30) for grafting of polystyrene onto silica and polymeric materials (e.g., polyglycidyl methacrylate (PGMA), polythiophene, polypropylene, and polyacrylate).
However, ATRGP and RAFT pose unique constraints. For example, ATRP requires a precise initiator-to-catalyst-to-monomer ratio, optimal temperature/solvent conditions, and surface-bound organic halide initiators, which potentially limits the surface graft density. RAFT graft polymerization requires thio-ester surface initiators for grafting. On the other hand, NMGP relies on conventional peroxide initiators and/or thermal initiation to form polymer chain radicals that may then, for example, reversibly bind to an alkoxyamine for controlled polymerization.
Plasma surface treatment has been proposed as an approach to alter surface chemistry and potentially supplant previous solution phase initiator strategies with high density surface activation. Plasma treatment alone, however, has been shown to be an insufficient surface modification tool; polymeric, plasma-treated surfaces do not retain their modified chemical properties over time and with air exposure. Vapor phase plasma polymerization, in which monomer fed through plasma is initiated in the gas phase and then polymerized on a substrate surface, has also been investigated as a surface modification method. However, surface-adsorbed radical monomer species, which are designed to polymerize with condensing monomer radicals from the vapor phase, may in fact be further modified by continuous plasma bombardment, leading to highly cross-linked, chemically and physically heterogeneous polymer films that are non-covalently adsorbed to the surface. Also, the local concentration of monomer species in the plasma afterglow is highly dependent on the radial dimensions of the plasma source, and the resulting spatial variations in monomer deposition rate may lead to a non-uniform film structure and morphology.
Plasma-induced graft polymerization (PIGP) is an alternative surface modification approach in which plasma is used to activate the surface, and ethylenically unsaturated monomers in the liquid phase are sequentially grafted to the initiation sites via a free radical grafting mechanism. This approach allows one to engineer a grafted polymer phase characterized by a high surface density of polymer chains that are initiated and polymerized directly from the substrate surface, thus minimizing polydisperse chain growth, and improving stability under chemical, thermal and shear stresses. Given the complex surface chemistry and limited lifetime of reactive plasma initiated surface species, the exact chemical nature of these plasma-generated organic moieties is yet to be established.
To date, PIGP has focused primarily on low pressure (i.e., below atmospheric) plasma initiation and surface grafting on polymeric materials. An example is low pressure polystyrene surface grafting used for surface structuring of Nafion fuel cells and separation membranes. Limited studies of low pressure plasma surface treatment of inorganic oxides, such as titanium dioxide, have also been reported. However, restrictions associated with low pressure plasma processing (e.g., the need for a vacuum chamber) are a hindrance for potential scale-up opportunities in industrial applications.
A notable limitation for achieving PIGP on inorganic substrates, unlike polymeric materials, has been the requirement of a sufficiently dense layer of surface activation sites, created through silylation of reactive monomers or macroinitiator grafting, that may form surface radicals for polymer initiation upon plasma treatment. Surface preparation required for such techniques combined with the reliance on surface hydroxyl chemistry limits the large-scale adaptation of such methods and the level of chain density that can be achieved. Direct plasma initiation and grafting without the use of surrogate surfaces has been demonstrated qualitatively on titanium oxide particles and silicone rubber materials, with characteristic surface radical formation noted as a function of treatment time and RF power, similar to organic materials. Yet, a recent study has demonstrated that, under low pressure plasma surface treatment of Shirasu porous glass, a direct correlation between silanol density and grafted polymer density is observed. This suggests that the number density of surface radicals that may be achieved in low pressure plasma surface activation of inorganic oxide substrates may be limited by the native oxide surface chemistry.
These findings, combined with the added requirement of ultra high vacuum chambers necessary for low pressure plasma processing, indicate that prior art approaches are insufficient for achieving high-density surface activation and graft polymerization, and especially inadequate for large surface area modification of organic and inorganic substrates.